Clathrate Hydrate Crystal Growth in Natural Gas Saturated Water Flow

Abstract

The growth of clathrate hydrate crystals in a flow of water saturated with simulated natural gas was visually observed. The simulated natural gas was a mixture of methane, ethane, and propane in a molar ratio of 90:7:3 or 98.5:1.4:0.1. The morphology (i.e., the shape and size) of the hydrate crystals that grew in the water flow changed depending on the system subcooling, which denotes the difference between the hydrate-equilibrium and experimental temperatures. At lower subcooling conditions, polygonal flat-plate crystals were observed. When the subcooling temperature was larger than 11.5 K, polygonal crystals were completely replaced by dendritic crystals. Crystals formed in flowing liquid water grew for a longer period of time than those in the quiescent system without any further guest supply. In addition, the coexistence of structure I and II hydrates with the 98.5:1.4:0.1 gas mixture in the continuous supply system was visually confirmed. From these observations, we note that the crystal morphology also depended on the crystallographic structure of the hydrate. These results enable us to precisely evaluate the hydrate morphologies corresponding to the marine sediment conditions.